This study provides a new, detailed, and fully three-dimensional structural interpretation of the active convergent plate boundary beneath the outer Nankai accretionary wedge offshore Kii Peninsula in SW Japan. Structural interpretation was done using the 12 by 56 km 2006 Kumano 3D seismic reflection volume, with the goal of providing new insights on the structural, temporal, and spatial evolution of the shallow plate boundary megathrust. Results of the structural analysis suggest three decollements with complex, three-dimensional geometries that have played a role in constructing the outer Nankai wedge in the study area, only one of which shows recent activity. Based on comparison to sandbox models and the presence of a modern, partially subducted seamount, their development and evolution are linked to past and present subduction of oceanic crust with signicant relief, including the Paleo-Zenisu Ridge east of the study area. Other characteristic features of the outer wedge likely developed from the effects of basement relief subduction as well, including the low velocity zone, along-strike variation of imbricate thrusts, prominent along-strike ridges across the margin, and several laterally discontinuous strike-slip faults. Evidence that the outer wedge evolved through several phases of basement relief subduction has strong implications for the mechanical properties of the deeper, seismogenic portions of the megathrust. Other studies suggest subduction of basement relief correlate with low slip-decit rates, VLFs, and slow slip, and may act as a barrier to coseismic rupture. Therefore, the results of this study can be applied to convergent margins with similar tectonic settings around the world to better understand the link between basement relief subduction, decollement and wedge evolution, and seismogenic behavior.